This application claims the priority of German Patent Application No. 198 33 644.6, filed Jul. 25, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a reactor unit in a system for producing hydrogen-rich gas from a liquid raw fuel.
For the operation of fuel cells with polymer membranes (called xe2x80x9cPEM fuel cellsxe2x80x9d) for mobile applications, a hydrogen-rich gas can be produced from a liquid raw fuel, especially methanol, for example, by the reformation of steam:
CH3OH+H2Oxe2x86x923H2+CO2.
In this process, carbon monoxide is also produced as a byproduct in amounts which damage the PEM fuel cell. The reformer product gas must therefore be cleaned in a gas cleaning stage. Selective oxidation on a suitable oxidation catalyst is known as a possible approach to a solution. The oxidation of CO is highly exothermic.
In the course of a chemical reaction, it is often advantageous to establish a temperature change through the reaction zone. In the case of the highly exothermic CO oxidation, the CO content is reduced, resulting in lower CO concentrations. Thus, selectivity becomes poorer. It is also true that higher temperatures result in poorer selectivity. By lowering the temperature level in the rear portion of the reaction zone of a CO oxidator, the selectivity can be substantially improved. In the case of the endothermic reformation reaction, a reduction of the initial CO concentration can be achieved by a low temperature in the rear portion of the reaction zone of a reformer. The thermodynamic equilibrium of the exothermic water-gas shift reaction plays a part, which shifts toward greater CO concentrations at higher temperatures.
In known reactor systems for water-vapor reformation of hydrocarbon or CO oxidation, a single reactor is used at uniform temperature, or structurally separate reactor units are used to achieve the different temperature levels.
In the first case, no optimum adaptation to the reaction temperature is possible. In the second case, the result is a high volume and mass requirement.
In GB 922259, a reactor unit for the reformation of methanol or for the CO shift reaction is described in which the addition of steam for the shift reaction takes place outside of the actual reaction chamber in a plurality of heating chambers connected to the actual reaction chamber.
When water is added, the water is brought into direct contact with the reaction medium issuing from the reaction chamber, so that steam is formed, which is absorbed by the reaction medium. The reaction medium is then fed back into the reaction chamber to an additional catalyst bed. The addition of the water abruptly chills the reaction medium. This chilling can be utilized in controlling the temperature of the reaction at the catalyst bed.
A very similar reactor unit for the CO shift reaction is disclosed in DE 197 19 997 A1 in which several chambers separated from one another in the direction of flow are present into which water is injected. The choice of the water temperature can be used to control the temperature of the catalytic reaction.
In EP 0 529 329 A2, a reactor unit for the CO shift reaction is disclosed in which waste heat is carried away by means of two tempering media. The reaction chamber for the shift reaction is sandwiched between the chambers through which the tempering media flow.
It is an object of the present invention to create a reactor in which the reaction temperature can be optimally achieved for the particular reaction, thereby achieving a low volume and mass requirement.
This object is accomplished with a reactor unit according to the present invention. According to the present invention, two tempering chambers separate from one another and from the reaction chamber are integrated into a reactor unit of laminate construction, and are associated with different areas of the reaction chamber. Thus, two reaction zones, which can be held at different temperature levels, are formed within the reaction chamber.
The two tempering chambers can be constructed both as energy sinks and as energy sources regardless of the requirements of the course of the reaction. As an energy sink, the tempering chamber can be referred to as a cooling chamber. As an energy source, the tempering chamber is a heating chamber.
In one embodiment of the present invention, one tempering chamber carries a liquid tempering medium and the other tempering chamber carries a gaseous tempering medium, so that the two tempering chambers are part of a cooling or heating circuit.
In an alternative embodiment, a catalyst material can be present in the tempering chambers. By a special chemical reaction (endothermic or exothermic) on this catalyst, a specific temperature level can be achieved.
With the reactor unit according to the present invention, optimization of the reactor temperatures to the course of the reaction is achieved. Also, the magnitudes of different temperatures in the two reaction zones can be varied as needed.
The integration of two reaction stages, which formerly were structurally separate, in a single structural unit makes the reactor more compact. By constructing the reactor unit in a laminate design, the mass and volume are reduced by two end plates in comparison to the known, structurally separate reaction stages. At the same time, the reactor unit according to the present invention entails a significant cost reduction compared with the known reactors.
The reactor unit according to the present invention makes no great demands of the technique of its manufacture and therefore can be made simply and inexpensively.
The reactor unit according to the present invention can be used within a system for the production of a hydrogen-rich gas from a fluid raw fuel, especially for the reformation of hydrocarbons or water-gas shift reaction, or in the gas purification stage of the system for selective carbon monoxide oxidation. The reactor unit of the present invention can be used especially in a PEM fuel cell system for mobile applications.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.